I did not mean to imply that sex is not important or intimate. It is both!

My point was that we often ascribe the act with a certain meaning, which is often wrapped around our beliefs, and this can lead to interpretation.

As an example of what I'm trying to say, let's look at someone who believes that sex should only take place between two people who love each other. Therefore, when sex happens, the act helps confirm to them that their partner loves them. Sex, to this person, has grown to mean love. That can lead to the interpretation that if their partner has sex with someone else, they don't love them anymore. Yet we all know that love can happen without sex and sex can happen without love.

Another belief: that romantic and sexual relationships only have value if monogamous. Therefore, to that person, the idea of a romantic, sexual partnership means monogamy. A variant on that is the belief that when people are sexual and committed to each other, they "belong to" each other, a sort of ownership.

If a partner wishes to cultivate romance with someone else, or have sex with someone else, or both, then these meanings that have been given to their current relationship can lead to interpret the partner's desires or actions as meaning the relationship has less value to them, or it means that they have been "lost" somehow and no longer belong to them.

If one is operating out of both these beliefs, it is easy to feel slighted, unloved, left behind, betrayed, and all that painful stuff when a partner says, "I want a romantic sexual relationship with someone else outside of our relationship." Jealousy often sits on top of other deeper feelings, but there tends to be an element of feeling something that was yours was taken from you. But these are all mostly mental constructs.

Hence, my assertion that sex holds whatever meaning we give it. I thought maybe that would be helpful to the OP in looking at the feelings coming up at the idea of his partner being sexually involved with someone else. It's always good to examine our beliefs and the meanings we give things in our lives. Not saying it is wrong to do that, but that through awareness we can deconstruct how we've set things up in our minds and find ways to handle the impact of issues that bump up against these beliefs and values we've created for ourselves.